Voice operated control circuit



April 26, 1949. HERRICK 2,468,553

VOICE OPERATED CONTROL CIRCUIT Filed March 17, 1945 GOI L BALANCING q HYBRID CIRCUIT INVENTOR.

ROSWELL H. HERRIGK ATTORNEY Patented Apr. 26, 1949 I 2,468,553 VOICE OPERATED CONTROL CIRCUIT .Roswell H. Herrick, Chicago, 111., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application March 17, 1945, Serial No. 583,240

6 Claims.

The present invention relates generally to improvements in signal current transmission systems of the type in which signal controlled switching circuits are provided for enabling certain of the signal current channels. included therein which are normally partially or completely blocked and, more particularly, to improvements in telephone substation circuits having incorporated therein coupled signal current channels for the transmission of incoming and outgoing signal currents.

. In theusual telephone substation circuit a hybrid system or anti-sidetone impedance network is provided for independently coupling the transmitter and the receiver to the line extending from the substation. In this arrangement one of the factors which determines the efilciency of the sidetone suppression is the impedance of the talking circuit established by the connection of two subscribers lines. This impedance is not the same for any two established connections and, accordingly, the hybrid system of each substation is usually balanced to provide maximum sidetone suppression for average line conditions. In installations wherein amplification of incoming and outgoing signal currents is required, as, for example, in executive loudspeaking sets, the conditions of unbalance introduced in the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. This is particularly true in substation installations provided in an exchange area where the subscribers lines are of widely different lengths. In order to completely obviate the singing which may result due to unbalance of the substation circuit occasioned by unfavorable line conditions, signal controlled switching means may be provided in the substation circuit for enabling the channels which are normally blocked. Such an arrangement is disclosed in my prior Patent No. 2,282,405, granted May 12, 1942, wherein gaseous discharge tubes are employed for controlling the gain of the signal current amplifiers. In this prior patent a gaseous discharge tube is associated with each channel so that the transmission efiiciency of the channel may be controlled-by the space current of the associated tube which-has its input electrodes coupled to the channel so that a predetermined start potential is impressed on the start electrode of the tube when signal current traverses this channel. Since the start electrode loses control after the tube has fired, circuit means are provided in each channel for lowering the anode voltage of the associated gaseous discharge tube when the flow of signal current through the corresponding channel ceases, thereby to deionize the tube and restore control theredf to the start electrode. In one embodiment a relay and auxiliary vacuum tube are employed to effect the deionization of an. associated one of the gaseous discharge tubes when the signal currents cease flowing, and in a second embodiment a source of alternating current is employed as the anode current supply for the gaseous discharge tubes. In the present invention an improved arrangement is employed wherein the voltage drop across a gaseous discharge tube is employed to enable an associated signal current amplifier, said gaseous discharge tube being fired in response to signal currents applied to the input of the amplifier, and wherein avacuum tube, also controlled by the signal currents applied to the input of the amplifier, is connected in series with the gaseous discharge tube and a source of direct current in such a manner as to quench the arc discharge therein when the signal currents cease. Since the arc voltage is inherently constant and free from voice components the need for filters having large time constants, such as are usually employed in voice operated gain control systems, is eliminated. In arrangements of this nature it is common for the supply voltage source mentioned heretofor to vary as much as 20% and thereby cause severe variations in the amplifier gain control circuit. The gaseous discharge tube is therefore arranged to also maintain the potential supplied to the amplifier tube at a constant value independent of source variations or signal input potential variations.

It is an object of the present invention to provide an improved transmission system of the character described wherein the magnitude of the control voltage which is utilized to affect the amplifier gain is stabilized by means of a gaseous discharge tube independent of variations in the source potential and signal input potential.

.It is a further object of the invention to provide an improved transmission system in accordance with the preceding object and wherein the gaseous discharge tube is provided with a control electrode upon which signaling currents are im pressed in order to initiate a discharge in the tube at a lower anode potential than would be required in the absence of the control electrode.

Other objects of the invention will appear upon a further perusal of the specification taken in conjunction with the accompanying drawing which illustrates a substation circuit having inamplifier at a point ahead of'the blocked" tubeand having its anode and cathode connected to a source of direct current in series with a load resistor, the voltage drop across the load resistor being employed to alter the biasing potcntialimpressed on the normally blocked tube in the amplifier when signal currents are impressed on the amplifier input terminals, and a gaseous discharge tube connected across the load resistor i t'o'stabilize the voltage drop thereacross and to eliminate voice components therefrom while signal currents are being transmitted. In addition; the gaseous discharge-tube is preferably providedvvith a control electrode which is also coupled to the amplifier at a point ahead of the normally blocked tube whereby a discharge will be initiated in the tube at a lower anode potential than would otherwise be possible.

Referring now to the drawing, the substation circuit there illustrated is shown connected to a telephone line I which may, for example; form a part of a conventional automatic or manual telephone system, in which case the distant end thereof will terminate-in a line circuit the char acter of which is determined by the character of the-exchange. The substation circuit comprises a transmitter or microphone 2" and receiving means in the form of a loudspeaker 3 which are coupled to line I through an anti-sidetone im'-- pedance network including a, hybrid coil 4' and a' balancing circuit 5' by means of signal transmis' sion channels 6 and 1, respectively. The outgoing signal current channel 6 comprises a three'stage' amplifier including the amplifier tubes 8, 9; and

H], the first of which receives signal'cur'ren'ts-from the microphone 2 and the last of which delivers the amplified signal currents to line I through hybrid coil 4. The first and last stages of thisamplifier are conventional, each employing cath"-- ode biasing. The cathode resistor ZB'for tube B-isconnected in series with potentiometer across gaseous discharge tube 22 for a purpose to be described later.

The second stage is the one Wherein the gairr of the amplifier is automatically controlled and this will be described in greater detail. The amplifier tube 9 employed in the second stage may have either a sharp or a remote cut-01f characteristic. A sharp cut ofii'characteristic is pref--- erable When the-amplifi'er'is' to be normally-completely blocked, while a remote cut-01f characterist'ic is preferable when the amplifier is to beriormally" only partially'bloclre'd. In either: case, the: cathode of tube 9 is connected to the tap on: a. voltage divider l I connected across the plate supply. The tap on divider l l isadjusted so that the biasing voltage in the input circuit of tube 9 is suchas to provide the desired minimum, or zero, normalgain. In order to change the value of the bias potential impressed on the control grid. of tube 9, an auxiliary vacuum-tube I2 is provided. Tub-e l 2 is capacitatively coupled to the output of amplifier tube 8; The grid of tube I2 is biased to the cut-off point by means of battery l3 where'- by this tube operates as a plate detector; that is, only the positive half cycles of a signal on the grid are amplified by the tube. When sound waves impinge on transmitter 2, the corresponding speech signals produced thereby are first amplified by amplifier tube 8 and then further amplified and rectified by control tubev I 2. The space currentiof tube l-Tflows ithrohgli resistors; l4 and 23' in series, a portion of'wliich is connected in series with the grid circuit of amplifier tube 9, thereby producing a voltage drop which acts to lower the bias potential impressed on the control gridof amplifier tube 9. The increased voltage drop" across resistor 23, produced by the rspaceacurrentof'tube l2, increases the cathode potential of tube 2'4so as to block channel 1. The average valueof the'space current of control tube l2 varies in accordance with the magnitude of the speech signals impressedon the input of the amplifier. In order to prevent the voltage drop across resistor" [4 from varylri'gaccordingly; and also to eliminate the voice? components which would otherwise" appear" thereacross, a? gaseous discharge" tube 5 is connected across resistors It and 23. This gaseous discharge tube mayhave but two elements, in" which case'thevoltage across resistors I 4 and 23" would be permitted to rise until it reached the striking point of the gaseous discharge tubeandwouldtlien'dropto'a constant value equal to that across the-gaseous discharge tube when ionized. As is well known; the" voltage drop across the gaseousdischarge'"tuba. when ionized; is inherently constant and is a function of the particular gas with which the tube is filled, the gas pressure, and the'electrode" shape and spacing; Thusa constant" current will flow through resistors Wand 23 and the ex"- cess space current of tube 12' will flow through tube [5. Condenser l6 ls'c'onnectedbetween the cathode and anode of"tube I'-2 to prevent eXtinc-'- t'ion' of the gaseous dischargetube during" the negative half cycles of"signal" current. Thusdur ihg" the positive-halfcycles condenser Iii-partially discharges through" control tube f2; and during the negative half cycles condenser i6 partially charges in series with" tube l5 in multiple with resistors M- and" 23 'from the pl'ate supply; Condenser I6 is made as large as possible consistent with the require'dspee'd ofarc' establishment. It should be noted that it is not necessarythat con denser I6" be so large as to substantially'eliminate voice components across resistor M by filter action' for this function is performed by'the gaseous discharge tube. The'timedelayin' enabling usually produced as a consequen'ceof' the necessityof employing filte'rs to eliminatevoicecomponentsfrom the control voltage is thussubstantially eliminated. The gaseous dischargetube willalso prevent any ripple or hum present in the power supply system due to inadequate filtering from appearingacrossfre'sistor' I142 Condenser I1 isconnected acrossresist'ors I 4 and 2 3 tomaintain:

the cathode of. control? tube: l at roundpotential for" speech frequencies prior to the initiation o'ff'ai discharge in tube" f5? Since" the voltage" required to initiate-"a1 di's'-*- charge in a two elec"erode gaseous discharge tube is considerably greater than the voltage drop therein after the discharge has been initiated; tube 15 is preferably" provided with a control electrode which is shown" capacitat'ively' coupled to the output circuit of amplifier tube 82 A battery I8 is connected so" as to impress a biasing potential on the control electrode of tube IE. The: signal voltage derived" from the output of" amplifier tube 8 serves to initiate a. discharge between the control electrode and cathode of tube l5. This discharge is transferred to the anode at a much lower anode voltage than would be required to initiate the discharge between cathode and anode without the control electrode. Once the discharge has transferred to the anode, the control electrode plays no further part in the operation until the signal currents cease, whereupon the discharge between the cathode and anode will be interrupted due to the decrease in space current of tube l2 and the control of tube is is then once more restored to the control electrode thereof. A high resistance I9 is connected in series with the control electrode of tube l5 to limit the current flowing thereto from the cathode and also to avoid undue loading of the output circuit of amplifier tube 8.

The tap on potentiometer I4 is adjusted so that the net biasing potential impressed on the control grid of amplifier tube 9 is the optimum value for full gain when the voltage drop across the potentiometer M and resistor 23 in series is equal to the arc voltage of gaseous discharge tube IS. The input signal level at which a discharge is initiated in tube I5 and amplifier tube 9 is enabled may be controlled by adjustment of potentiometer 2D. The gain of the amplifier may be varied by adjustment of potentiometer 2| without affecting the input signal level at which enabling occurs.

Since the incoming signal channel I is identical to the outgoing signal channel 6, excepting that its input signal is obtained from the hybrid coil 3 over line I and its output is delivered to loudspeaker 3, it is believed that the operation thereof should be apparent from the preceding description of channel 6. When signals are being transmitted over channel 1, channel 6 is blocked by the increased potential developed across resistor 26 just as channel I was blocked by the increased potential across resistor 23 during the transmission of signal currents over channel 6.

While a particular embodiment or the invention has been described, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a transmission system, a signal current transmission channel including amplifying means, said amplifying means including a stage comprising an electron discharge tube having a grid electrode upon which a disabling potential is normally impressed, an impedance element, a source of direct current, a second electron discharge tube having its output circuit connected across said source in series with said impedance element and having its input circuit coupled to said channel at a point preceding said amplifier stage, said second electron discharge tube being biased to cut-01f whereby signal currents traversing said channel are rectified in said output circuit to produce a unidirectional potential across said element, said impedance element being connected to said grid electrode of said first electron discharge tube so that the potential produced across said impedance element opposes said disabling potential to thereby enable said amplifier stage when signal currents traverse said channel, and a gaseous discharge tube connected across said impedance element to prevent voice components from appearing thereacross.

2. In a transmission system as claimed in claim 1, a control electrode in said gaseous discharge tube also coupled to said channel at a point preceding said amplifier stage to facilitate the initiation of a discharge therein when signal currents traverse said channel.

3. In a transmission system, a channel over which signal currents are at'times transmitted, an amplifier in said channel adapted to have its gain controlled through variation 01 a biasing potential, an impedance element, a source of direct current, a vacuum tube having its output circuit connected across said source in series with said impedance element and having its input circuit coupled to said channel, said vacuum tube being biased to cut-off whereby signal currents traversing said channel are rectified in said output circuit to produce a unidirectional potential across said impedance element, said impedance element being connected to said amplifier so that the potential produced thereacross varies said biasing potential to thereby change the gain of said amplifier, and a gaseous discharge tube connected across said impedance element to maintain the magnitude of said unidirectional potential constant during variations of the potentials of said source and the signal input.

4. In a transmission system, a signal current transmission channel including an amplifier, a source of direct current, a space discharge device and a gaseous discharge device serially connected across said source, each of said devices having a control electrode coupled to said channel, said devices being normally nonconductive, an impedance element connected across said gaseous discharge device, said space discharge device being rendered conductive and thereby developing a potential across said impedance element in response to signal currents traversing said channel, said potential causing a discharge to be initiated in said gaseous discharge device, the initiation of said discharge in said gaseous discharge device being aided by the signal currents impressed on the control electrode thereof, said gaseous discharge device acting to maintain said potential constant during the signal transmission, the gain of said amplifier controlled by said potential.

5. In a voice operated control circuit, a source of direct current, a space discharge device and a gaseous discharge device serially connected across said source, a common circuit, each of said devices having a control electrode coupled to said common circuit, said devices being normally nonconductive, an impedance element connected across said gaseous discharge device, said space discharge device being rendered conductive and thereby developing a potential across said impedance element in response to signal currents traversing said common circuit, said potential causing a discharge to be initiated in said gaseous discharge device, the initiation of said discharge in said gaseous discharge device being aided by the signal currents impressed on the control electrode thereof, said gaseous discharge device acting to maintain said potential constant as long as the signal currents traverse said common circuit, said space discharge device being rendered nonconductive in response to the cessation of said signal currents thereby extinguishing the discharge in said gaseous discharge device.

6. In a telephone system utilizing a voice current repeater tube, voice controlled means for alternately rendering said tube effective and ineffective to repeat speech waves comprising a space discharge tube having an input circuit and was 7 an output circuit and means for applying voice *signals to said input circuit biasing means The following references are of=record in the mall rendering said second tube nonconductive, file of h a said space discharge tube energized in response to the application of said voice signals, an im- 5 pedance element in said output circuit energized Number, Nam-e Date, in response to the energizing of said space 6115- 2331405 Herrick May 2, 19 2 charge tube, said impedance element being con- 2 3 0 phaip t 23., 9

nected to said repeater tube so as to apply an enabling potential thereto when energized, and 10 a gaseous discharge tube having its anode circuit shunted across said impedance element in respect to said source to prevent voice components from appearing thereacross.

ROSWELL H. HERRICK. 15 

